In dyeing keratinaceous fibers, particularly human hair, it is desirable that the dyed fibers have a natural appearance. This often requires blending dyes that are primarily red, blue or yellow in color in the proper portions to provide the natural looking shade. A major problem is the general lack of yellow dyes having light-fast qualities comparable to such qualities in the red and blue dyes in the shade. As a result, blended dyes containing yellow are prone to shifts in hue with normal sunlight exposure.
It has been found, unexpectedly, that the presence of the quaternary group in the compounds of formula I above shifts the light absorbancy of these compounds to the shorter wavelength when compared with the corresponding unquaternized compounds. This shift provides useful dyes having a yellower color when dyed out on keratinaceous fibers, e.g., hair or wool. It has also been found, unexpectedly, that these quaternized compounds provide dyes that have an improved lightfastness when compared with the corresponding unquaternized compounds.